The present invention relates to a self-shielded type of coil unit and an MRI (magnetic resonance imaging) system using the same, particularly to a self-shielded type of coil unit that comprises a main coil for providing a utilizable magnetic field of a desired spatial distribution and a shield, or screen, coil for providing a shielding magnetic field substantially preventing the field from leaking out of the unit.
Medical magnetic systems have various types of coil assemblies. Such coil assemblies include a first coil assembly for generating a static magnetic field, a second coil assembly for generating a field that corrects the static field in uniformity, a third coil assembly for generating gradient magnetic fields superposed on the static field, and a fourth coil assembly for transmitting and receiving RF signals.
Of these, as the gradient coil assembly, various types of shielded coil assemblies have been developed for preventing gradient fields from leaking outward therefrom. A coil assembly called self-shielded gradient coil (frequently referred to as TSGC: Twin Shielded Gradient Coil or ASGC; Actively Shielded Gradient Coil) is one type falling into such self-shielded types. The self-shielded type gradient coil assembly comprises two distinct components for each of X-, Y-, and Z-channels: a main, or primary, coil that provides pulsed gradient fields of spatially desired distributions; and a shield, or screen, coil disposed so as to surround the main coil. The shield coil is used to provide a shielding field that prevents most magnetic flux provided by the main coil from leaking onto surrounding conductive structures placed around the coil assembly. In consequence, gradient fields leaked from the coil assembly decrease, suppressing the influence of eddy currents due to the leaked field on images.
The efficiency of shielding obtained by the self-shielded gradient coil is decided by the number of turns of a shield coil (i.e., the number of windings). Both the main and shield coils should be connected to power supply taking into account demands from the configuration of power supply circuitry. Under such conditions, a preferred technique for connecting coils is that both the main and shield coils are mutually connected in series to receive in common current from a single power supply. When adopting such series connection, the number of turns of the shield coil is determined on the turn-arrangement relationship with the main coil. The efficiency of shielding is thus determined depending on the number of turns. It is impossible to raise the efficiency of shielding any more under such configuration.
To overcome this difficulty, a technique for raising the efficiency of shielding by altering the number of turns of a shield coil is proposed by a paper"practical aspects of shielded gradient design, Barry L.W. Chapman, SMRM, 943,1995." In this proposal, special conditions different from how to use inductance elements in the ordinary electric circuit are imposed on the coils. That is, for changing the number of turns of the shield coil, the amount of current flowing through the shield coil must be controlled. Specifically, a dummy resistor or shunt power source is coupled with the shield coil in parallel so that the current though the shield coil is controlled.
However, when using a dummy resistor for current control as above, the magnetic characteristics of the self-shielded gradient coil may change, because the resistance of the dummy resistor changes with changed temperatures. The efficiency of shielding realized by the shield coil is also fluctuated or decreased.
In particular, where current is changed time-dependently, the amounts of impedance are mutually changed between one branch circuit into which the shield coil is inserted and the other branch circuit into which the dummy resistor is inserted. This will cause the current through the former branch circuit with no shield coil inserted to be reluctant to flow. Therefore, for scanning on a pulse sequence requiring current pulses at a higher rate (for example, an ultra fast scanning sequence such as an EPI (echo planar imaging) method), following changes in current is difficult for magnetic fields generated from the shield coil, causing the waveforms of the generated magnetic fields to be curved. A configuration in which the dummy resistor is connected is not proper for scanning that has been getting faster and faster recently.
On one hand, for using the shunt power source connected in parallel with the shield coil, the greater a n amount of current providing to the shunt power source gets, the larger the size of the power supply to the assembly becomes, raising cost in production. It is also true that the configuration using the shunt power source is inappropriate for scanning on ultra fast pulse sequences, because response times thereof are relatively longer.